The A40 is clearly a thermal switch but no data can be found for this Type. The envelope clearly shows that it was made by Ediswan for the British Post Office or GPO and has the serial number of 1309. Telecommunications were part of the GPO remit and a delay switch is used with mercury vapour rectifiers.
The heater is a tungsten coil placed a short way away from the bi-metallic strip moving arm of the switch. At four volts and 0.65 Amps the filament burns yellow white but even after several minutes the moving arm did not move. The heated strut can be seen to be insulated from the moving arm by the ceramic bead.
The balloon envelope is 45 mm in diameter, and excluding the B4 base pins is 97 mm tall.
Updated February 07, 2015.